1. Field of the Invention
The embodiments disclosed relate generally to non-destructive testing and more particularly to ultrasound inspection methods for noisy materials and related probes.
2. Description of the Related Art
Casting is widely used in many industrial applications to manufacture intricate and expensive parts. As understood by those of ordinary skill in the applicable arts, castings are manufactured in rough forms and further machined into final shapes, which in many applications require high-quality sealing surfaces obtained through detailed final machining cuts. However, inherent to casting processes are shrinkage, inclusions, or voids in the cast part created at least in part by dirt or sand coming off from the mold or material shrinkage during solidification. As such, when rough cast parts are machined into final products, many times subsurface areas of shrinkage, inclusions, or voids are reached, generating a defect that, at a minimum, has to be repaired, or, at a worst case scenario, requires that part must be scrapped.
In the Oil and Gas industry, for example, casting is used for the production of large industrial component manufactured for use in oil and gas production and transmission systems. There are generally three cast materials used for these components—nodular cast iron, flake cast iron, and cast steel. Depending on the type of cast material used and the location of the defects discovered, the cast materials may be repaired or may be scrapped for remelting, thus contributing to an increase in manufacturing cost due to either the repair cost and/or lead-time required to replace scrapped castings.
X-ray, eddy current inspection, magnetic particle inspection, liquid dye penetrant, and ultrasound are various examples of commonly used techniques to inspect cast materials. However, these conventional techniques are limited in their ability to detect small subsurface defects. In liquid-dye-penetrant techniques, a die is spread over a surface to be inspected and excess dye material is wiped off, leaving only that portion of the dye that was absorbed in a defect. A powder absorbing material is then used to locate the surface defect. Similarly, in magnetic-particle inspection, magnetic particles are spread over the surface to be tested and a magnetic field is applied, causing the particles to concentrate in an area where the magnetic field leaks are caused by the defect. In view of their very nature, neither liquid-dye-penetrant nor magnetic-particle inspection techniques can be used to detect small subsurface defects in cast parts.
In eddy-current inspection, circulating (or eddy) currents generated at the surface and near surface of a part being inspected are perturbed by defects and detected by the eddy-current inspection system. However, as understood by those of ordinary skill, small defects are difficult to detect deep below the inspection surface with eddy currents because the skin effects quickly attenuate the energy getting into the surface and because the effective area increases with depth, thus increasing the size of the smallest defect that can be detected. X-ray techniques use a radiation source that penetrates through the thickness of the test piece to record defects on a X-ray detector placed opposite to the radiation source. However, the size of defects detected by X-ray inspection is limited to about 2% of the thickness of the sample, which thickness may range from 50 mm to over 300 mm thick. Thus, the ability to inspect thick parts with X-ray devices is significantly reduced. In addition, specialized high-energy X-ray facilities or gamma ray instrumentation are required for these types of inspection, thus limiting the number of casting suppliers that are capable to invest in expensive facilities to conduct high-energy, X-ray or gamma-ray inspection of large castings.
Ultrasonic inspection has been used for cast material inspection for many years. It is commonly considered that the resolution of defects when using ultrasound in cast materials is limited due to the nature of the microstructure of cast materials. Nodular cast iron and flake cast iron contain a great deal of carbon segregated from the iron. The segregated carbon in these materials will scatter the ultrasound leading to very noisy ultrasonic signals. One of the challenges to inspecting casting materials using ultrasound is to be able to discriminate defect signals from normal cast material microstructure scatter signals. However, conventional scattering theory teaches that the frequency of the acoustic energy should be decreased for the detection of defects in cast materials in thicker parts. Conventionally, dual element probes operating at frequencies between 1 MHz and 5 MHz are used to reduce the sensitivity to scatter and retain sensitivity to the defects, but their performance to detect defects having a characteristic length of about 0.5 mm is not satisfactory.
It would therefore be desirable to develop new ultrasound inspection methods and related probes for noisy materials with increased sensitivity to small subsurface defects, while maintaining or reducing the sensitivity to microstructure background noise.